You can quite easily see the difference between the
high-side configuration and the low-side configuration. In the low-side
configuration, the load is connected between the drain and +V, while the source
is connected to ground. Thus, the gate drive is referenced to ground. So by
applying a voltage of >7V (for Power MOSFETs) or >4V (for Logic Level
MOSFETs), the MOSFET can be fully turned on.

Now let’s talk about the high-side configuration. The load
is connected between the source and ground with the drain connected to +V. Thus
the gate drive is not referenced to ground as source is not connected to ground
and gate drive is related to VGS (voltage at gate with respect to source). I’ll
talk about this in a while.

The difference in operation for the MOSFET configured as
high-side switch as opposed to the MOSFET configured as low-side switch is
that, it acts as a current source whereas the MOSFET configured as low-side
switch acts as a current sink.

Now let’s go back to the high-side drive. Let’s say you
apply a voltage of 12V (with reference to ground) to the MOSFET gate. However,
when the MOSFET is on, voltage at source is equal to +V. Let’s assume +V is
+15V. Now the problem is +12V gate drive (with reference to ground) will not
keep the MOSFET on. When the MOSFET is on, the MOSFET source will be at a
potential of +15V. To be on, the MOSFET must have +8V VGS minimum. So, if
source is at +15V, the voltage at the gate with respect to ground must be at
least +23V. If source was at +300V, for example, gate drive would require a
minimum of +308V with respect to ground. This is if the gate drive is
referenced to ground. If you have a separate isolated power supply whose ground
and the ground of the MOSFET-based circuit are isolated, thenyou can use that to drive the MOSFET as well.

There are quite a few ways to drive MOSFETs in high-side
configuration. The first thing that might come into the minds of many of you
would be a boost converter circuit or a charge pump circuit to use as the drive
voltage for gate drive. This concept is sometimes used and isn’t wrong.
However, it is usually used when voltage gap between control circuit voltage
and gate drive requirement is small. If you needed to step up voltage from 12V
to 40V for example, you might be able to accomplish it quite easily. However, a
problem arises when there is the need to step up voltages from 12V to, say,
300V. In such situations, other solutions must be sought.

One simple solution is to use a gate drive transformer. This
is not a method I prefer and thus I will not talk about it here. If you are
interested, I might write another article just for gate drive transformers.

Another
solution is to use a separate/isolated power supply whose ground is
separate from the ground of the MOSFET-based circuit. See Fig. 3 below.

The other popular method is to use bootstrap based drive. In
this drive method, a capacitor is charged to the required VGS – let’s say 10V –
when the MOSFET is off. Then this capacitor is used during driving the MOSFET
to provide an additional 10V over the source.

First I’ll talk about the use of a separate/isolated power
supply. Here’s a circuit diagram illustrating this:

When a logic high is given to “Drive Signal”, potential at
optocoupler pin 4 (emitter of optocoupler transistor) is about +12V with
respect to the ground / negative terminal/point of BAT1 – the separate/isolated
power supply. This point is connected to Q1 source. Thus Q2 turns on. About
+12V, with respect to Q1 source, is provided to Q1 gate. Thus Q1 is driven on.

When a logic low is given to “Drive Signal”, optocoupler pin
4 (emitter of optocoupler transistor) is at the same potential as the ground /
negative terminal/point of BAT1 – the separate/isolated power supply. So Q3
turns on and pulls Q1 gate low. Thus MOSFET Q1 is driven off.

Note that the optocoupler ground is the same ground as the ground of the MOSFET-based circuit.

Here’s the current flow for when the “Drive Signal” is a
logic high.

Fig. 4 - Current flow when driving a high-side N-channel MOSFET on, from a separate/isolated power supply (click image to enlarge)

This driver can be used for any duty cycle – all the way
from 0% to 100%. The driving frequency is limited by the speed of the
optocoupler. For high frequencies, optically isolated MOSFET drivers may be
used instead of the two transistors and the optocoupler – the optically
isolated MOSFET driver will be all that’s needed. Some such drivers are TLP250,
TLP350, HCPL3120, etc.

Now let’s talk about the boostrap based drive. Here
when the high-side MOSFET is off, a capacitor is charged from the
driving voltage. The capacitor charges through the load or a supporting
low-side MOFSET. When the high-side MOSFET is to be turned on/driven,
the voltage on the capacitor is used to drive the high-side MOSFET. Thus
the limitation of this method is quite obvious. A large enough
capacitor should be used for storing the required energy/charge for
keeping the high-side MOSFET on for the required time. At the same time,
the capacitor must be large enough that during the entire driving time,
the voltage doesn't fall below about 8V, in order to prevent the MOSFET
from being only partially on. Thus, the bootstrap based drive can not
be used for 100% or close to 100% duty cycle. And the lower the
frequency of operation, the larger the required capacitance.

The easiest
way to drive a MOSFET using the boostrap based drive is to use a dedicated high
side MOSFET driver. Some drivers come with just the high-side driver while many
come with both high-side and low-side drivers. IR2117, for example, is one
driver that contains a single driver that can be used to drive a high-side
MOSFET driver. IR2110, which is arguably the most popular high-low side MOSFET
driver, features a high-side driver and a low-side driver in a single device. I’ve
written a detailed tutorial regarding the use of IR2110. Here's the tutorial:

Besides the IR2110, there are many high-low side drivers
available. L6385E and NCP2181 are my personal favorites. You can choose from a wide range of high side and high-low side drivers.

No
matter which method you choose, once you know how to handle the drive
requirement, it's really easy. In most cases, I use the bootstrap based
drivers, although I do occasionally use isolated power-supply based
drive. None of these methods are too difficult and I hope I've managed
to provide you a clear answer to your question: "Why are high-side N-channel
MOSFETs to be driven differently from low-side N-channel MOSFETs, and how do we
drive the high-side N-channel MOSFET?" Do let me know your comments and feedback.

Use MOSFETs with high enough voltage rating. Also, make sure you take care of any spikes and transients. Eg if you have a motor, when the motor turns off, if you don't have any freewheeling diode/circuitry in place, the spike from the motor will damage your MOSFET.

HI, i have implemented the circuits for testing high and low side, as you have shown on others links for ir2110. I am getting correct signal LO relative to the LIn. But at HO I am getting constant 12v for square wave Hin of 6v at 1khz. where i am doing mistakes ???

To measure the signal at HO, what you need to do, is to remove the drive connections to the MOSFETs and connect VS to ground. Then check the signal at HO. If you see the correct signal, remove the connection from VS to ground and then connect the MOSFETs.

It is due to the virtual ground (VS) that your oscilloscope does not properly detect/display the signal.

I need to generate 600 V dc from a 12 V battery. So I want to use a square wave MOSFET inverter to convert 12 V dc to 12 V ac. I am not using sine-wave inverter because I don't have to run any appliances. After that I will use a high frquency ferrite transformer to get 600 V ac from 12 V ac . Finally I will rectify & filter it to get 600 V dc.

Can I use PWM to control the voltage of the square wave inverter? The output of inverter goes to transformer. Is it okay for the transformer to get a PWM input? If yes then what should be the range of duty cycle of PWM?

Yes you can use PWM to control the voltage of the square wave inverter. Yes, it is ok for the transformer to get a PWM-ed signal. In fact that's what ferrite transformers get all the time. You should be good with a frequency between 30kHz and 60kHz. The duty cycle will vary depending on your transformer winding ratio and will control the output voltage.

To control the output voltage using PWM, take feedback from the output side (remember to convert to DC and scale down).

i want your help, i am designing an smps in half bridge, just like the old AT smps not having any auxilary supply, i want to use pulse transformer to drive Mosfets, mosfets are IRF840, power is approx 250 w 28V @ 9 - 10Amps for battery Charging .

Thabk you so very for your explanation on how to configure mosfets on low and high side..God bless you:can you please provide me with information regarding the construction of a 3 kva inverter using sg3425 and mosfets...thanks:-)

Great stuff, best I have found :) Wondering just one small thing...On the opto isolated drivers like the TLP250 you mentioned, Is that all you need is the power supply to the driver and the driver does the "bootstraping" internally? Or do you need an external bootstrap or voltage boosting in addition? Thank you

If you have an isolated driver with the isolated power supply, no "bootstrapping" occurs. Bootstrapping is done only when there is no isolated power supply - to provide a "new" or "virtual" ground with which to reference (if thinking like that helps).

The isolated power supply provides the gate voltage (with respect to source) required to drive the MOSFET.

Hello Tahmid,I would like to ask about the configuration when using HCPL3120. I have tried supply the 3120 with isolated source, R2 = 10R, R3 = 1k, and the 2 transistors were skipped, the mosfet I used was IRFP460A, VCC for the mosfet is 24V and the load is 270ohm just for demo. Unfortunately, the voltage between drain and source pin was not square like the input, it had the straight rise-up and down but had curve like sin wave at the top and not stable. I think that is the result of not sharing the same ground (or I have done something wrong). Could you help me a little bit? And what happens if the optocoupler ground is not the same ground as the ground of the MOSFET-based circuit?

The voltage rating of the isolated source I used for the HCPL3120 was 24V, because the VCC - VEE has to be bigger than or equal 13.5V. And the ground I mentioned was the ground of the cathode of the HCPL3120. Can it be different from the ground of the source that supplies to the drain terminal?

I have tested with Optocoupler like 4N35 and TLP521-2 and the circuit worked fine with the frequency below 2kHz. But if I raise the frequency up, it starts to curve on the way downI would like to work with higher frequency from at least 25kHz to 300kHz, that why I used HCPL3120. But the HCPL gave out the waveform with the top happening to be like a sine wave and not very stable.

I want to drive IRF840 at the frequency of 22Khz when load is connected and at lowest possible frequency (say about 250Hz) when load is disconnected. Please suggest me exact values of bootstrap components or give me the calculations.

I'm not an expert on electronics, though I do small things with atmegas and such.

I'm willing to build and test this circuit (together with some of your circuits on low-side switching back in your other post), to build a three-phase bridge to control a photocopier's a brushless motor with an atmega (I know I should add diodes in series with the nmos to cancel voltage spikes from the windings).

I just wanted to confirm that `BAT1' can be replaced by a 220v->12v transformer + rectifier bridge + smoothing capacitors + lm7812, and wanted also to ask if `BAT1's circuit is actually isolated, as it connects to the load under the nmos (wouldn't it's ground be then connected to the load's ground?).

In my case, I will initially test the circuit with 24V/3A, but would also like to use it in a future with an AC induction motor (220V) I have laying arround (very small, like 3/8 HP, I'd say), so just checking I won't fry anything.

Thanks, and I wish you all the best, and that you keep sharing knowledge as your studies/life allows you so.

hii used ir2117 to drive mosfet for dc dc buck converter the input for the driver is pwm from microcontroller i attached the circuit but there is a problem the output from this driver is 9v pure dc not pwm like the input http://www.edaboard.com/thread310188.html#post1327298need your help thanks

Use one per MOSFET. The two high side MOSFETs should have separate isolated power supplies while the low side MOSFETs can have the same power supply. Alternatively, you can create a capacitor-based bootstrapping-based driver for the high side MOSFETs and only one power supply. Alternatively again, you can create a separate charge-pumped (boosted) voltage to use to drive the MOSFETs - this is easier to achieve for low voltage drives compared to high voltage drive.

The circuit is complete. There are just different current paths. Remember that the MOSFET is an insulated gate device. So, there's a current path to drive the gate. And there's another current path which is the main high current path.

hi ,im doing a circuit design on proteus . it is a dc-dc converter circuit and it is feedback using PIC16f877A to drive the pwm to the mosfet. but i really face difficulties in driving the mosfet and i came across to know optocoupler. i think mayb optocoupler might help to drive the mosfet. since i totally new in optocoupler (i just knew it today), can i send you my circuit and really appreciate for your help in figuring how should i make the mosfet work perfectly for my circuit. thanks!

Do you want to control both speed and direction? If so, you'll need the full-bridge configuration. Do keep in mind that you can't go to high duty cycles with a bootstrap-based high-side driver. In that case, it'll be better to use an isolated power supply, or a charge pumped higher voltage power supply to drive it.

If all you need is speed control, you can just have the MOSFET work in the low-side configuration with the motor between drain and supply voltage. Remember the anti-parallel diode across the motor.

thank u Tahmid for ur explanation but i want u to draw a pratical circuit using component on how to get that isolated power supply Bat1 to run the totem drivers. Thank u for ue help because this high side drive is really a big problem to me sir. Thanku

If you're running off of the main power supply, you can use a transformer isolated push-pull converter to generate multiple isolated voltages on the secondary. Rectify and filter these at the output and regulate if necessary. Then you can have as many isolated supplies as you need.

I've sent an email to you about my question but you have not still responded it yet :) Anyway,i have questions for you :

1) I have 24V 3 Phase BLDC motor 120W i tried to control it with IR2110 but i did not understand when i applied to 1V to its VDD pin it draws approximately 500mA !! Most probably it shorted out somewhere in the circuit but did not find :S Also, i need almost full range of pwm for speed control so should i use ir2110 for driving this motor ?

2) Or, i have tlp250. Should i use it for driving ? I have msp430f series which has output 3.3V and as you said in different subject that at least 4V supply is suitable for IR2110 series to arrange the logic threshold but you did not suggest this way. Therefore, i want to use tlp250 but is it required using external BJT as you used in Fig 3. ?

3) When i will use 48V 1.2kW motor this tlp250 method is still applicable ? Also, i want to replace high side fets into IGBT ?

4) Why did not you use diode parallel with 10ohm resistor placed in gate as you used in your IR2110 application ?

5) Should i use flyback diode or snubber circuit or diode i don't know exactly what the purpose does it use with my bldc H-bridge circuit ?

6) Should i use parallel electrolytic or other type capacitor between high side N-fet drain to Low side N-fet source actually between the power supply vcc to gnd ? If yes, should i use it for each column (3 column 1 high 1 low to drive 3 phase) ?

I want to explain each question briefly im sorry if i exaggerated the explanations :S

Hello, I use IR2110 to control power mosfet to power a three-phase brushless motor. I have to run the engine for a start. I mounted a charge pump that provides a voltage of 30V to VB. VCC = + 15 = Vmot= + 24 Every time I turn on the circuit the engine will not start: I lightly touch the tree to start working.Can anyone help? thanks Luciano

hey man , just a quick question :D , I am building this circuit : http://www.lammertbies.nl/electronics/PI_metal_detector.htmland i was planning on using mcp1407 but had no luck finding it, i could only find the ir2110, but have no idea whether to use it in high or low side config. do you have any idea what config. would work by looking at the schematic ? the gate driver is supposed to be connected to an arduino , thanks !

One huge advantage in using N-channel MOSFETs as opposed to P-channel MOSFETs is that the conduction losses are much lower for similarly priced devices. Another disadvantage of P-channel MOSFETs is the much lower availability and higher cost at high voltages.

Your article has helped me a lot already, I'd like to use dual N-Channel MOSFETs as a high side AC switch. Can I ask, could the 'BAT1' supply be a switching power supply 230VAC --> 12VDC which is powered from the main supply that will be switched. (i.e. does a switching power supply provide enough isolation from the main supply to be switched)

Hi Tahmid Am working on DC-DC converters, in my converters two mosfets are there, one is high side and one is low side. I have to turn on two mosfets with same duty simultaneously is that possible by IR2110. Am generating pulses from TL494 and giving similar pulses to Lin and Hin. will i get same pulse at Lo and Ho.

I want to use IR 2110 to drive one low side MOS and one high side MOS. However, the gating signal for both the switches will be same. That means when the low side MOS is "ON" the high side MOS will be "ON" as well and vise-versa. Would you kindly help me in this regard?Can any one help me on this

hello tahmid, my issues now with this design is that i dont want to use an isolated power supply because i am using the circuit as an Solar MPPT charger. please help with the non isolated high side driver. thanks

How would you approach designing a buck converter with Vin = 1170V and Vout = 370V. I am using a SiC high side FET and am having issues finding a good combination with a gate driver. what do you suggest?

what are the other high and low side drivers apart from ir2110 for driving a simple h-bridge inverter(full bridge)??..My circuit details are input voltage 36VLoad resistance 20ohms LC filter values 200micro henry and 10 microfarad

Hi,I a Mechanical Engineer, currently working on Regenerative braking with BLDC.Trying to use N channel mosfets as high side switches. Being new to electronics, its really messing my mind haw to make that bootstrapping capacitor circuit.Isnt there any earier option? Like cant I but the bootstrapping circuit completely with capacitor fixed to it, so that I can plug it on before my Power mosfet?

Need serious help, and urgently.Need to take and show readings within 3 days..Any help is highly appreciated.

Hi Thamid, I looked at your post and took great help. I understand how this bootstrap circuit works, but i 've some problem in the practical realization, so i hope you can help me by answering these questions. In particular i'm using L6385e which you said to be your favorite choice:1) Could I use it to drive single N channel MOS of buck converter which drive rotor and stator (two buck) of dc motor?2)I read on your other post in a forum but i am not sure to understood that could be problem in this configuration, in particular referring to the fact that the load should be shorted to zero potential to allow capacitor's charging, so there is something i must be aware?

Hello Tahmid, I am having dificuilty in driving the H-Side N-Channel MOSFET using Optocoupler Based Gate Driver ACPL-33KT of Avago Technologies Using Isolated Power Supply, the low-side MOSFET turns fully ON for Vgs about 8V for all power voltages , but when the power side voltage is increased above 12V, we find that the Vgs is only 5V and even then the Pulse is intermittent, when we increase Optocoupler Vcc , Vgs is increasing but Optocoupler Max Vcc is 20V, So can you suggest if bootstrap circuit is necessary even if we are using isolated supply for High-Side Gate Drive, and how to select Bootstrap Capacitor, is it just a dependant on Qgate Value from MOSFET Datasheet

hello guys, i just have a question that i need help with. i have an inverter that outputs a 240 V ac signal. i need to display the power thats output on an LCD. do i need to make use of current and voltage sensors and if yes, how and where do i use them and to what do i connect them. thanks in advance

hi sir tahmid. i am working on my program to drive a single phase induction motor but my problem is that i cant generate a sine wave output at the inverter part. i am using an opto coupler as a driver circuit and an n-channel mosfet. can you help me out with a circuit?

Hi Tahmid, thank you for sharing your knowledge. I'm pretty new to this, so i most probably missed something. Can you please tell me how Q3 turns on. To me it looks like if the signal is low, the opto is open. How is Q3 going to conduct then. Thanks

Hi .. tahmid , Iam using IR2117 to drive the mosfet in buck converter , At no:2 pin i have given a PWM signal with avg voltage of about 6 volts and max about 12 volts , but at pin 7 or 6 there is no output .. what could be the possible problem ???

Hi TahmidI have one problem during implementing the same circuit.The circuit works well when i am using 12 volt battery to supply VDD but circuit does not work when i connected 220 volt dc to mosfet vdd.Why is we have to tied ac neutral to rectified voltage but in case of bridge rectifier it is not possible.Help me please i am in great trouble or share your email id so that i can mail you my problem.Thanks

Dear Tahmid i use TL250 to drive 1200v Module iGBT in my Buck converter for 360 v Batter charger , but some failures of IGBT and i see TL250 heating up a little more, what do u suggest mr Tahmid, ur suggestion will be higly appreciated

hi Tahmid, my name is Marco Petrolesi from italy, i've problem with a mosfet driver. I am using IR2117 to drive N mosfet (IRF540N) to build my buck converter.At pin2 i have given a square wave 100KHZ from PWM signal generator with 9 volts pk to pk.At pin 7 there is no output.The converter is powered by 48V power supply on the drain of mosfet, while source is connected at pin 6 of ir2117, at fast recovery diode and inductor, ecc.ecc.Nothing to do! no signal output!Sometimes only for 1/4 second i can see signal... after, nothing.Can you help me, please!I ask if you can write to my @mail address: petrolem2012@gmail.comtankyouMarco Petrolesi

Mr.Thamid... I had formed a full bridge using IR2110 driver IC and IRF840 Mosfet to Drive a motor of 24vdc at 1khz PWM frequency for a duty cycle from 10-90% it operates fine even at 72vdc, but I face a problem, every time High Side mosfet burnt out and remaining all 3 Mosfets also damaged when I gaves a 320vdc.I connect 2x220v incandescent lamp as a load accross a full bridge output. PWM is generated by PIC 16F886. Please help me out to solve the problem.

Hi Tahmid, I have been trying for a week to drive a 3 phase ac bridge using six N channel Mosfets (IRF1404) and 3 IR2111 drivers, My high side mosfets gates are connected to 12V, and the drivers Vcc are 12 Volts as well. I am using a pic micronctroller 18F1330 with three pwm outputs with 120 degrees phase angle. I used a NPN transistor to raise the PWM output to 12 volts as the IR2111 is threshold is above 6.5 volts, however I think the bootstrapping part is the major problem that I am having as I have very little knowledge about bootstrapping. I am using 0.1uF ceramic capacitor for Vcc and gnd, and 0.1uF ceramic between Vb and Vs in parallel with a 1uF (I also tried 10uF), and 1N4007 diode between Vcc and Vb. My PWM switching frequency varies between 50 Hz to 3KHz.. I am also connecting 1K resistors between Source and Gate for all mostfets.

I am sure of my driver connectivity schematic, but I can't make it work. Do you have any clue what can be wrong?

Great explanation.I need a suggestion from you, Im using H-bridge to chop 5v DC, if I use a 12v supply with respect to ground for the gate, it must be sufficient to drive as its more than 5+4(vgs threshold voltage) please reply

I realize copying an integrated high-side driver seems like it's the best way to implement this type of circuit, but for many if not most implementations, you can eliminate the bipolar transistors and several resistors. The gate on a MOSFET draws only a tiny amount of current. If you want to prove this, leave the gate of an MOSFET open, put one hand on the GND of your circuit, and the other touch the gate, the MOSFET will turn off, now switch from GND to the positive side, touch the gate, and it will turn on (perhaps slowly if there is any residual capacitance.)

So to do this, eliminate Q2, Q3, R4, R5, R6. Now take U1 pin 4 directly to R2. When the input LED to U1 is 'off', the output transistor of U1 is inactive and R3 pulls the gate of Q1 low. Turn on the Optocoupler, and R2 gets pulled high, and the MOSFET is turned on. Les parts, and very reliable, I've used it this way in several projects.

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About Me

I am Syed Tahmid Mahbub, from Dhaka, Bangladesh, born on August 1, 1994.
Electronics is my passion and from class V, I have been learning electronics. I learnt and worked mostly on SMPS, power electronics, microcontrollers and integration of microcontrollers with SMPS and power electronics. I've used PIC and AVR microcontrollers - PIC 10F, 12F, 16F, 18F, 24F, dsPIC 30F, 33F, PIC32, ATmega and ATtiny, integrating them with various SMPS and power electronics circuits.
I have completed my Bachelor's degree from Cornell University (Class of 2017) in Ithaca, New York, USA, majoring in Electrical and Computer Engineering (ECE).
I am a member of the forum www.edaboard.com, where I am an "Advanced Member Level 5" (the highest level attainable) and also the forum allaboutcircuits.com, where I am a "Senior Member". I post to help solve electronics-related problems of engineers and engineering students from all over the world.
I love watching and playing cricket and football (soccer), and listening to music.
I am now a hardware engineer at Apple in Silicon Valley, California, USA.